1. Field of the Invention
The invention relates to an arrangement for controlling an echo canceller and a centre clipper inserted in a telephone circuit comprising a receive path and a send path in which an echo signal may be generated by a signal entering the receive path. The echo canceller comprises an adaptive filter connected to the receive path; a subtractor arranged in the send path for subtracting the output signal of the adaptive filter from the signal in the send path; and an adjusting circuit for automatically adjusting the coefficients of the filter so the filter supplies an approximated value of the echo signal. The centre clipper is arranged in the send path at the output of the subtractor for suppressing there, when it is activated, signals having an amplitude equal to or below a clipping threshold. The coefficient adjusting circuit and the centre clipper operate under the control of said control arrangement, which forms control signals from the signal in the receive path and the level N.sub.REC of this signal and from the levels N.sub.AV and N.sub.AP of the signals in the send path, respectively before and after the subtractor.
2. Description of the Related Art
In the above-described echo canceller, the adaptive filter has for its object to supply as accurate a copy as possible of the echo signal appearing in the send path of a four-wire circuit, in response to the signal generated by a distant talker in the receive path and passing through an unwanted echo path between the receive and send paths. When the echo path does not comprise non-linear circuits capable of generating noise, the adaptive filter may form, after an adaption period, an echo copy signal which in practice enables the cancellation of the echo signal at the output of the subtracting circuit. But in certain cases, the echo path may be the seat of cascaded non-linear operations such as analog-to-digital conversions or PCM-to-differential PCM conversions and vice versa. In such cases, the accumulated quantization noise in the echo path degrades the operation of the adaptive filter for low levels of the echo signal and produces a prohibitive residual echo level at the output of the subtractor. This occurs, for example, in the case of long-distance links via satellites in which an echo canceller is provided near a ground station for cancelling an echo generated by an imperfection in a two-wire-to-four-wire transition located anywhere in a nation.
To obtain an attenuation of the residual echo which the actual echo canceller cannot suppress, the CCITT recommends to arrange in the send path, at the output of the subtractor, a non-linear processing circuit which is often referred to as a centre clipper: see on this subject Recommandation G. 165 and Report R.24, appendix 3, of Study Group XV. The centre clipper is a circuit which nearly suppresses the signals having an amplitude equal to or below a clipping threshold and transmits the signals having an amplitude above this threshold. The centre clipper associated with an echo canceller has for its object to suppress the residue of the echo not eliminated by the echo canceller in single talk conditions of the distant talker so as to ensure that the latter does not hear his own echo and to disturb to the least possible extent the near talker speech signal to be transmitted via the send path.
Associated with the centre clipper is a control circuit to adjust the clipper continually to the state most suitable for performing its function, namely: to an active state when only a residue of the echo is present at its input, and to an inactive state corresponding to the transmission of all the signals in all the other situations, more specifically in double talk periods or when the echo is insufficiently eliminated by the echo canceller. To fulfill these conditions correctly, the centre clipper must respond rapidly to all changes in the conditions.
The operation of the coefficient adjusting circuit is also controlled by a control circuit, essentially: for authorizing the correction only when the output signal of the subtractor (error signal controlling the correction) is significant relative to the receive signal level; for disabling the correction in double talk conditions; for setting the coefficients to zero after divergence of the coefficients has been detected; and, possibly, for instantaneously increasing the correction step after the coefficients have been set to zero.
In certain types of known echo cancelling arrangements (see, for example, the article by Horna published in COMSAT Technical Review, Vol. 7 No. 2, 1977, pages 393-428), the control of the centre clipper is of a design similar to the control circuit of arrangements known as "echo suppressers" and is realized independently of the echo canceller. This control is based on the comparison between the level N.sub.REC of the signal in the receive path and the level N.sub.AV at the input of the send path. When the level ratio N.sub.REC /N.sub.AV is high, it is assumed that only the distant talker is speaking and producing an echo to be eliminated, therefore the centre clipper is made operative. When this ratio falls below a threshold, it is assumed that only the near talker is speaking or that a double talk condition exists, therefore the centre clipper is adjusted to the inactive state. The weak point of this criterion resides in the fact that the signal from the distant talker, present in the receive path, produces at the input of the send path an echo which may be variably delayed according to the echo path. When the signal from the distant talker decreases (at the end of each syllable), a significant echo may still be present in the send path, so that the ratio N.sub.REC /N.sub.AV gives the false impression that a double talk condition exists which makes the centre clipper inoperative at an inopportune moment. To try and remedy this shortcoming, one might delay switching the centre clipper from its active to its inactive state. This will, however, result in a degradation of the speech signal from the near talker when this talker wants to break in on the conversation during said delay. Finally, when the ratio N.sub.REc /N.sub.AV is used, it is only possible to detect strong double talk and the near speech signal will be distorted in weaker double talk conditions.
In an echo cancelling arrangement described in European Patent No. 0 053 202, some of the above-mentioned shortcomings are obviated by also taking into account the level N.sub.AP of the signal after the subtractor of the echo canceller, whilst the control of the centre clipper is combined with the control of the coefficients adjusting circuit. These controls are determined by a simple logic combination of three criteria which are obtained by respectively comparing the three ratios N.sub.AV /N.sub.REC, N.sub.AP /N.sub.REC and N.sub.AP /N.sub.AV with three thresholds. A disadvantage of this system is its lack of flexibility. Thus the three above-mentioned thresholds, which determine the operation of the echo canceller and also of the centre clipper, must be fixed at values which are chosen as a compromise and which may be detrimental in certain circumstances to the correct operation of the echo canceller. Similarly, the combinatorial character of the control logic makes it difficult to provide appropriate temporizing of certain controls, which may be vary useful in certain circumstances.